An anxious Boden lies still in the MRI scanner, knowing he has to wait 24 hours until he's officially diagnosed with lung cancer. Herrmann finds the chief crossing names off the 1985 roster. In a moment of vulnerability, Boden confesses three men from the list have died due to asbestos exposure.
Severide confronts the young woman he saw Benny laughing and chatting with over lunch - does she know Benny has a wife and kids? The woman stares strangely at Severide and drops an emotional bomb: she's Katie, Benny's 21-year-old daughter. Severide begins to put the pieces together: Benny left Severide and his mother to fend for themselves about 21 years ago...
Clarke stomps out of the firehouse, angry that his estranged wife, Lisa, wants to speak with him. She left him when he needed her most. Lisa apologizes for bailing but insists Clarke treated her like garbage. Despite their issues, she wants to reconnect with her husband.
Summoning his courage, Chief Boden addresses his firehouse for the last time. It's been an honor serving with them; he knows the house will be in good hands, which is why he's retiring. Firehouse 51 is stunned by the news, but before they can verbally respond, the emergency siren sounds.
The dazed firefighters pile into the fire truck, expecting a routine call. However, they're greeted by a derailed train, dozens of injured and trapped passengers and the root cause: a rogue tanker filled to the brim with liquid propane. Boden barks initial orders, instructing Dawson and an inexplicably distracted Shay to set up triage. Due to McLeod's cutbacks, Firehouse 51 is fighting this battle without any backup.
Casey helps a man with a severely snapped leg hobble to triage, promising him he'll find his missing daughter. Clarke cuts through a jammed door and frees dozens of passengers. Clambering into the car, he watches a man twisting a corkscrew into another passenger's head to relieve pressure. The man, Westin, is an Army medic and he's not leaving the injured behind. He and Clarke are working fervently to assist passengers when Westin suddenly collapses, trying to conceal a massive bruise on his side.
Triage experiences a stroke of luck: David, a local hospital's chief of trauma, happened to be driving by the accident. Rolling up his sleeves, David rallies the harried Dawson and Shay, making the difficult decisions about whom to save. Clarke protests when David places Westin on the fatal black tarp, but he's beyond saving. His tough Army attitude kicks in with Clarke holding his hand as Westin breathes his last.
Mills and the chief have just charged into the neighboring warehouse for two potentially trapped workers when two tons of debris pen them inside the building. The propane tanker's release valve breaks, forcing Boden to call it - all personnel need to pull out and allow the able-bodied to evacuate. It's only a matter of time before the tanker blow the block to kingdom come. The firefighters refuse to allow Boden's last mission to end in catastrophe. Charging back in, they free him and Mills.
Genius strikes Severide and he hooks up the squad's broken water hose to the tanker, funneling the propane a safe distance from the flames. Mills chucks a flare at the hose, allowing the propane to catch fire and burn off. The immediate danger dissipates and Firehouse 51 wraps up their assignment. David hands Dawson and Shay his business card, complimenting them on their extraordinary work.
Later, McLeod greets the team, pleased with herself that her system "worked." Chief Boden corrects her: his team is well-trained, but they were left to roast today. They ran out of supplies and were severely understaffed. They escaped by the skin of their teeth today but maybe not tomorrow. During his exit interview, Boden refuses to back down to McLeod's bullying. He will not leave his house vulnerable and he doesn't give a damn what she says.
Benny apologizes to Severide for being a crappy father. He knows he screws over women. He's going back to Beth and the boys - Firehouse 51 is Boden's. Meanwhile, Boden receives the much-anticipated "all-clear" from the doctor. He's officially cancer-free. Clarke knocks on Lisa's door to apologize for being a jerk earlier.
Casey proudly flashes a hotel brochure at Dawson and explains he intends to take Griffin and Kevin fishing. Suddenly, he receives an unexpected call from Heather - she's been released early due to overcrowding! She has decided to start a new life for her and the boys and has taken an apartment management job in Florida. Casey envelops Griffin and Kevin in a bear hug, saying goodbye possibly for the last time.
The Firehouse 51 crew bites off more than they can chew with a train derailment.